A feature that was introduced in Past games is the implementation of Swarms. These are where Pokémon in certain routes have their appearence Percentage raised so that they are more common.

In Diamond & Pearl however, after you get the National Pokédex, if you go to your Other Rival's house in Sandgem Town and talk to their sister, they will tell you that there is an increase of Pokémon in a certain Route or area

If you go to the route or area that the girl states, within one or two wild Pokémon battles you will find the Pokémon at the same level as the other Pokémon.

The Pokémon and Route change every day and all of the Pokémon that you find and their evolutions are not obtainable by any other means in Diamond & Pearl so be sure to capture them while you have the chance.

Below are all the Swarming Pokémon and the routes/areas they swarm in: